Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging, “logging while drilling” (“LWD”), drillpipe conveyed logging, and coil tubing conveyed logging.
One example of a logging tool is a wireline dielectric tool. Dielectric tools determine the dielectric constant and conductivity of downhole formations from the real and imaginary parts of the complex propagation constant of electromagnetic waves traveling through the formations. By measuring the phase difference and amplitude ratio between two points in the formation, the tool determines the formation resistivity and dielectric constant. These measurements are useful for finding water-filled porosity, and water saturation can be computed if formation porosity is known. If multiple water saturation measurements are available (e.g., from different types of logging tools), it becomes possible to measure characteristics of the flushed zone.
Dielectric tools utilize sensors to evaluate the dielectric constant of the formation (5≤ϵr≤80). In order to obtain a good signal-noise-ratio (“SNR”), a high gain of coupling from transmitter to receiver is desirable. In practice, however, it is very difficult to achieve high coupling gain over the whole range of the dielectric constant of the formation. During operation, a dielectric sensor usually works well over a narrow range of the dielectric constant, because the matching between formation and sensor is good within the range. Nevertheless, the sensor's performance significantly deteriorates as a result of the out-of-range variation of the dielectric constant of formation, which worsens the matching. As an example, conventional high frequency dielectric tools exhibit good coupling gain and SNR for those dielectric constants between 30≤ϵr≤50, but its performance significantly deteriorates once the dielectric constant is outside of this range (i.e., 5≤ϵr≤30 & 50≤ϵr≤80).
Therefore, there is a need in the art to provide improved dielectric sensors which provide high coupling gain over the entire range of the dielectric constant.